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## Calculating g on Moon

An astronaut on the moon wishes to calculate the local g in the area. He sets up a wire with a mass of 1.7 grams and hanging from it an object with mass 4.98 kg. He sends some kind of pulse down the wire and calculates that it takes 40.86 ms to traverse the wire. What is g? You can neglect the mass of the wire when calculating the tension.

That was everything that was given in the problem. I have not a clue how to solve it. What I did was just use the equation of T=2pi[sqrt(l/g)] to solve for g and hope that the wire acts like a spring or something.

I’m totally sure that’s incorrect, but hey I’m not an astrophysicist.

(Response from Members and visitors awaited)

Posted by Ashish Dharva

## Change in resistance of a wire on stretching

A WIRE OF LENGTH L AND AREA OF CROSS SECTION A HAS A RESISTANCE R. IF THE WIRE IS STRACHED TO TWICE OF ITS LENGTH WHILE A REMAINS SAME , WHAT IS THE NEW RESISTANCE????

We know that R = ρl/A

When the wire is stretched, there is no change in volume. So if length is doubled then volume will be halved so that volume is constant.

Therefore, the new resistance R’ = 4R

That is the resistance becomes four times its original value.

1. A WIRE OF LENGTH L AND AREA OF CROSS SECTION A HAS A RESISTANCE R. IF THE WIRE IS STRACHED TO TWICE OF ITS LENGTH WHILE A REMAINS SAME , WHAT IS THE NEW RESISTANCE????

## Why we specifically note resistance in a circuit while all part of a wire have some resistance..?

Why we specifically note resistance in a circuit while all part of a wire have some resistance..???

The resistance of the rest of the circuit compared to the resistance of the resistor is negligible.

## Force on Current carrying conductor

“Thin and flexible wire is kept on a table in the form of a rectangle and a strong current is send through it. How will be the shape of the wire as a result?”

Answer: The parallel sides are carrying currents in opposite directions and therefore there will be repulsion between them. So, the rectangular coil will try to expand and tend to become circular.

## Is it true that a person touching a high power line get struck with the line?

Is it true that a person touching a high power line get struck with the line?

Ya, This is due to the strong contraction of the muscles making the person not at all able to release the wire or object from which he got the shock. Not due to any type of attraction arising out of electricity.

## Wire Gauge and Resistance problem

I am using 18 gauge nicrome wire having ohm of 17.3 , 220 volt. I want to use 10 gauge wire how to decide the length?

18 gauge corresponds to a diameter of 1.22 mm

10 gauge means diameter is 3.25 mm

Sine the resistance needs to be the same, we can use the formula

$\LARGE \bg_white R=\frac{\rho l}{A}$
Which gives
$\LARGE \bg_white \frac{l_{1}}{A_{1}}=\frac{l_{2}}{A_{2}}$
As resistivity is same.

$\LARGE \bg_white l_{2}=\frac{l_{1}A_{2}}{A_{1}}=\frac{l_{1}r_{2}^{2}}{r_{1}^{2}}$

Substitute the values and you will get the answer.

## A few selected questions fro CBSE Class 10 Physics Summative Assessment 1

Dear students,

If you are preparing for the CBSE Summative Assessment I for Class X, Give the following questions a try.

If you are not able to solve any of it, please post your doubts as comments to this post. The questions can be discussed here with due explanations.

Make it a practice to solve as much Sample papers and Guess papers as you can before writing the Board Exam.

All the best

1. An electric geyser has the ratings 2000W, 220V marked on it. What should be the minimum rating in whole number of a fuse wire, that may be required for safe use with this geyser? (1Marks)
2. Consider a circular loop of wire lying in the plane of a table. Let the current pass through the loop clockwise. Draw the direction of magnetic field inside and outside the loop. (2Marks)
3. A wire is stretched so as to make its length double. What is the effect on its resistivity and why? (2Marks)
4. Define electric power. Write its SI unit. What is the value of 1 kWh in joules. (2Marks)
5. A torch bulb is rated 3.0V, and 600mA. Calculate its (i) power (ii) resistance (iii) energy consumed if this bulb is lighted for 4 hours.  (3Marks)
6. (i) Differentiate between AC and DC giving two points of difference. (ii) What is the frequency of AC and DC in India? (iii) Draw a graph of current – time for AC and DC. (3Marks)
7. Name and state the law which relates the potential difference in a circuit to the magnitude of current that flows through it. Give its mathematical expression and draw the theoretical graph too. (3Marks)
8. What is electromagnetic induction? State Fleming’s Right Hand Rule. A coil of copper wire is connected to a galvanometer. What would happen if a bar magnet is (a) pushed into the coil with north pole entering first? (b) pulled out of the bar magnet? (c) held stationary inside the coil?
9. State Joule’s law of heating. How does the electric fuse prevent fires due to shortcircuit or overloading? A household uses the following electric appliances. (i) Refrigerator of rating 400W for ten hours each day. (ii) two electric fans of rating 80W each for twelve hours each day. (iii) six electric tubes of rating 18W each for six hours each day. Calculate the electricity bill of the household for the month of June if the cost per unit of electric energy is Rs. 3.00. (5Marks)

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